Tuesday
2/16/16
1. Number paragraphs pages 343-349 (21)
2. Read paragraphs 1-15. Highlight main idea.
3.
4.
Answer questions on pages 342-346.
Write a poem, short story or a rap about Democritus, Dalton,
Thompson, Rutherford and the Cloud Model. It should
include :
-what they did
-thought/proved
-the order in which they occurred…….
Have fun with this…..
HISTORY OF THE ATOM
460 BC
Democritus develops the idea of atoms
he pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
ATOMA
(greek for indivisible)
Early Greek Theories
Democritus
• 400 B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a void.
fire
earth
Aristotle
air
water
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
• Aristotle was wrong. However, his
theory persisted for 2000 years.
Early Modern Ideas
1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thomson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive
nucleus.
4) Bohr Model (1913)
Atoms have specific orbits
Each orbital has fixed energy
HISTORY OF THE ATOM
1808
John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them
ATOMS
John Dalton
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on reason.
•
•
•
•
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
HISTORY OF THE ATOM
1898
Joseph John Thomson
found that atoms could sometimes eject a far
smaller negative particle which he called an
ELECTRON
HISTORY OF THE ATOM
1904
Thomson develops the idea that an atom was made up of
electrons scattered unevenly within an elastic sphere surrounded
by a soup of positive charge to balance the electron's charge
like plums surrounded by pudding.
PLUM PUDDING
MODEL
Early Models of the Atom
Thomson
• Plum pudding model
• Atom made of a
positively charged
material with the
negatively charged
electrons scattered
through it.
HISTORY OF THE ATOM
1910
Ernest Rutherford
Oversaw Geiger and Marsden carrying out his
famous experiment.
They fired Helium nuclei at a piece of gold
foil which was only a few atoms thick.
They found that although most of them
passed through. About 1 in 10,000 hit
Ernest Rutherford
• Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance path of invisible
-particles
Most particles passed through.
So, atoms are mostly empty.
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
HISTORY OF THE ATOM
gold foil
helium nuclei
helium nuclei
They found that while most of the helium nuclei passed
through the foil, a small number were deflected and, to their
surprise, some helium nuclei bounced straight back.
HISTORY OF THE ATOM
Rutherford’s new evidence allowed him to propose a more
detailed model with a central nucleus.
He suggested that the positive charge was all in a central
nucleus. With this holding the electrons in place by electrical
attraction
However, this was not the end of the story.
Early Models of the Atom
Rutherford
• Mostly empty space
• Small, positive
nucleus
• Contained protons
• Negative electrons
scattered around the
outside
HISTORY OF THE ATOM
1913
Niels Bohr
Studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by adding
that the electrons were in orbits, like
planets orbiting the sun.
Each orbit only able to contain a set
number of electrons.
Early Models of the Atom
Bohr
• Electrons move in
definite orbits
around the nucleus
The Neutron
• The neutron was not discovered until 1932
when James Chadwick used scattering
data to calculate the mass of this neutral
particle.
• Since the time of Rutherford it had been
known that the atomic mass number A of
nuclei is a bit more than twice the atomic
number Z for most atoms and that
essentially all the mass of the atom is
concentrated in the relatively tiny nucleus.
Modern Model of the Atom
The electron cloud
• Sometimes called
the wave model
• Spherical cloud of
varying density
• Varying density
shows where an
electron is more or
less likely to be